xref: /freebsd/usr.sbin/traceroute/findsaddr-udp.c (revision 350b7c3570aa6c87c537e54f706f1866f93a4142)
1 /*-
2  * SPDX-License-Identifier: BSD-2-Clause
3  *
4  * Copyright (c) 2010 Bjoern A. Zeeb <bz@FreeBSD.org>
5  * All rights reserved.
6  *
7  * Redistribution and use in source and binary forms, with or without
8  * modification, are permitted provided that the following conditions
9  * are met:
10  * 1. Redistributions of source code must retain the above copyright
11  * notice, this list of conditions and the following disclaimer.
12  * 2. Redistributions in binary form must reproduce the above copyright
13  * notice, this list of conditions and the following disclaimer in the
14  * documentation and/or other materials provided with the distribution.
15  *
16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19  * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26  * SUCH DAMAGE.
27  *
28  * $FreeBSD$
29  */
30 
31 #include <string.h>
32 #include <unistd.h>
33 
34 #include <sys/types.h>
35 #include <sys/socket.h>
36 
37 #include <netinet/in.h>
38 
39 #include "findsaddr.h"
40 #include "traceroute.h"
41 
42 /*
43  * Return the source address for the given destination address.
44  *
45  * This makes use of proper source address selection in the FreeBSD kernel
46  * even taking jails into account (sys/netinet/in_pcb.c:in_pcbladdr()).
47  * We open a UDP socket, and connect to the destination, letting the kernel
48  * do the bind and then read the source IPv4 address using getsockname(2).
49  * This has multiple advantages: no need to do PF_ROUTE operations possibly
50  * needing special privileges, jails properly taken into account and most
51  * important - getting the result the kernel would give us rather than
52  * best-guessing ourselves.
53  */
54 const char *
55 findsaddr(register const struct sockaddr_in *to,
56     register struct sockaddr_in *from)
57 {
58 	const char *errstr;
59 	struct sockaddr_in cto, cfrom;
60 	int s;
61 	socklen_t len;
62 
63 	s = socket(AF_INET, SOCK_DGRAM, 0);
64 	if (s == -1)
65 		return ("failed to open DGRAM socket for src addr selection.");
66 
67 	errstr = NULL;
68 	len = sizeof(struct sockaddr_in);
69 	memcpy(&cto, to, len);
70 	cto.sin_port = htons(65535);	/* Dummy port for connect(2). */
71 	if (connect(s, (struct sockaddr *)&cto, len) == -1) {
72 		errstr = "failed to connect to peer for src addr selection.";
73 		goto err;
74 	}
75 
76 	if (getsockname(s, (struct sockaddr *)&cfrom, &len) == -1) {
77 		errstr = "failed to get socket name for src addr selection.";
78 		goto err;
79 	}
80 
81 	if (len != sizeof(struct sockaddr_in) || cfrom.sin_family != AF_INET) {
82 		errstr = "unexpected address family in src addr selection.";
83 		goto err;
84 	}
85 
86 	/* Update source address for traceroute. */
87 	setsin(from, cfrom.sin_addr.s_addr);
88 
89 err:
90 	(void) close(s);
91 
92 	/* No error (string) to return. */
93 	return (errstr);
94 }
95 
96 /* end */
97